A Charge-pump can provide a high rise/fall rate (such as approximately 3.3 Volts/nanosecond) rail-to-rail clock signals to drive external capacitors of various circuit components. In conventional systems, an aggressive ground and shield design strategy may be preferred on a printed circuit board (“PCB”) to avoid charge-pump clock switching noise getting electrically coupled to other sensitive sensor inputs, especially through a phenomenon known as “ground bounce,” and other forms of electromagnetic discharge. Ground bounce is especially problematic for video inputs, reset level clamps (“RLCs”), and analog to digital converter (“ADC”) reference pins.
However, as a countervailing consideration, cost-driven PCB design, such as for a “scan-head” with a coupled charge-pump may employ a single ground plane. This typically has hampered performance, due to such considerations as PCB and circuit geometry limitations. Thus, even though the charge-pump may be internally physically isolated from the sensitive analog circuits on a die, switching noise of the charge-pump could easily deteriorate a signal-to-noise ratio (“SNR”) of the AFE due to such factors as external electrical coupling through ground bounce.
In one initial implementation, it was found that the SNR of an AFE deteriorated from 62 dB to 40 dB, when a Charge-pump was enabled. Further investigation, which involved varying the AFE sampled rate, revealed that noise on conversion data of the ADC is an inter-modulation product between the charge-pump clock and a sampled frequency of the AFE. This conclusion has also been supported by observing that charge-pump clock edges have been variously coupled to video inputs, RLC signals, and AFE reference signals.
One approach to decrease noise of a charge-pump is to increase a rise/fall time of the Charge-pump, such as from 1 nanosecond (“ns”) to 3 ns. This increase has not been found to provide a significant improvement on the SNR. Moreover, a further increase is not practically possible on a die, since a minimum clock rise/fall time is dictated by efficiency and shoot-through concerns of the charge-pump.
Therefore, there is a need in the art for a noise suppression in an AFE that employs a charge-pump that addresses at least some issues discussed above associated with the AFE and charge-pump.